Photolithographic masks for the fabrication of semiconductor components typically include light-influencing structures formed on a mask substrate (e.g., a quartz substrate).
A light-influencing structure is understood here to be any structure on the mask substrate which influences light of the photolithographic process in a specific manner in order to image a structure e.g. on a wafer.
Such a light-influencing structure may be highly absorbent, but may also be part of a halftone phase mask, whose light-influencing structure transmits light in part (approximately 6%) but simultaneously brings about a phase shift of the light by 180°. In principle, however, a reflective layer may also be regarded as light-influencing.
In the case of halftone phase masks, e.g., a molybdenum-silicon compound. (MoSi) is used as the light-influencing material. By utilizing interference effects in the phase shift, it is possible to achieve a larger process window during the wafer exposure and processing than would be possible with the use of an absorber which absorbs 100% of the light.
On account of the cost savings demanded and the technological competition that exists, there is a continuous trend towards shrinking structures on a chip. This requires a reduction of the detail sizes of the light-influencing structures on the photo mask with which the structures are produced on the chip.
As the structures become smaller and smaller, the light-influencing structures of the mask must be prevented from having defects. If, at one location, e.g., too little light-absorbing material is applied on the quartz substrate of the mask or too much absorber material is removed during the patterning of the mask, then too much light is transmitted and a bright defect arises on the wafer.
Although the problem of defects exists in principle in the photolithographic methods, it is the case that as the structures become ever smaller, the tolerances for defects also become smaller. For this reason, repair methods that work ever more finely are required.
Methods for repairing defects of light-influencing structures are known in principle. Thus, use is made of a gas-assisted deposition of a carbon film onto a defect with the aid of an ion beam installation.
In this case, use is made of customary ion beam installations with gallium as ion beams. Good beam shapes and the required beam intensity can thus be achieved. With the aid of the gallium ions, by means of gas-assisted processes, either excess absorber material is removed on a photolithographic mask (“gas assisted etching”) or missing absorber material is replaced by the application of a carbon film.
The gallium ions have a supporting function in this case in that they excite process gases present in order that either a chemical reaction with the absorber material of the photo mask (gas assisted etching) can take place or that carbon atoms from the gas phase can be deposited onto the mask as a carbon layer.
The gallium ions have the property of absorbing light in an intensified manner the shorter the exposure wavelength with which the photolithographic mask is exposed. Hitherto, this property has not been used for the repair of masks, in particular halftone phase masks, and has been regarded rather as a disturbing side effect.
Since the dielectric properties of the applied carbon film and of the light-influencing MoSi layer differ, the following disadvantage is manifested, which becomes apparent in an intensified manner towards smaller feature sizes and large-area defects: the light transmission at the defect location differs from that at undamaged comparison locations despite repair. This difference in light transmission is at the margin of the allowed specification in the case of small line widths (line widths on the photo mask of less than 440 nm).